This read-write INTEGER, set to enable(1) or disable(2), turns on or off automatic diagnostic testing of physical lines on the module, where enabling it means a line alarm temporarily suspends traffic in both directions and starts local loop-back testing, while disabling it means one direction's traffic isn't affected by an alarm in the other direction. An admin weighs this tradeoff between automatic fault isolation (at the cost of a brief bidirectional traffic suspension) versus preserving one-directional traffic during a single-direction alarm. A module carrying latency-sensitive traffic where any suspension is unacceptable might have this set to disable(2) to avoid the auto-diagnostic traffic interruption.